Articles made from electrically nonconductive plastic can be metallized by an electroless metallization process or alternatively by a direct electroplating process. In both processes, the article is first cleaned and etched. The etching is typically undertaken by means of chromosulphuric acid. The etching serves to make the surface of the article receptive to the subsequent metallization, such that the surfaces of the articles are well-wetted with the respective solutions in the subsequent treatment steps and the deposited metal ultimately has sufficiently firm adhesion on the surface.
After etching, the plastic is activated for the electroless metallization by means of an activator comprising a noble metal, and then metallized electrolessly. Subsequently, a thicker metal layer can also be applied electrolytically. In the case of the direct electroplating process, which does not need an electroless metallization, the etched surface is typically treated with a palladium colloid solution. Subsequently, the surface is contacted with an alkaline solution comprising copper ions complexed with a complexing agent to increase the conductivity. This step leads to the formation of a copper layer and hence to a metal layer on the surface of the article with elevated conductivity. Thereafter, the article can be directly electrolytically metallized (EP 1 054 081 B1).
A further possibility to provide a sufficient electrically conductive layer on the etched surface is to treat it with a metal ion solution and afterwards with a sulphide solution. These two treatment steps generate an electrically conductive metalsulfide layer on the etched surface prior to metallizing it by direct electroplating (EP 1 0010 52 A2).
As etching solutions based on chromosulphuric acid are toxic, the literature describes attempts to replace it with etching solutions comprising permanganate salts.
EP 202 57 08 A1 discloses a pickling solution and a pickling process for pickling ABS plastic surfaces or ABS polymer blend surfaces prior to a subsequent metallization of these surfaces. The pickling solution includes Mn(VII) ions and an inorganic acid. The pickling solution is free of alkali and alkaline earth ions which is achieved by generating the Mn(VII) ions by anodic oxidation of manganese(II) salts. Redox mediators like Ag (I), Bi(V), V(V), Mo(VI), Cu(II), Ti(II), Au(I), Fe(II), Ce(III), or Sb(II) ions may be added in order to increase the efficiency of the anodic oxidation. The absence of alkali metal permanganate compounds overcomes the problem of rapid self-decomposition of the acidic permanganate solution. The pickling solution of EP 202 57 08 has the disadvantages that it is laborious to prepare and the educts, the manganese(II) salts, are expensive as they have to be free of alkali and earth alkali metal ions. The pickling process and the metallization process including the pickling require additional equipment which renders the processes and the respective process lines even more expensive and laborious.
Although acidic permanganate solutions can be suited for etching plastics surfaces and sufficient adhesion strength between deposited metal layer and plastic substrate may be obtained, acidic permanganate solutions are unstable. When etching the substrate surface, the permanganate ions are reduced and the end-product is manganese dioxide. Manganese dioxide catalyzes the formation of further manganese dioxide and oxygen and leads to self-decomposition of permanganate. Thus, the etching effect of a freshly prepared acidic permanganate solution will be insufficient already after several hours. The acidic permanganate etching solution can be re-sharpened by addition of permanganate which causes a high consumption of the permanganate compounds. In addition, the concentration of manganese dioxide further increases. Moreover, manganese dioxide forms a precipitate that is difficult to remove from the etching solution. As a result of this enrichment the viscosity of the etching solution will be increased up to a point where the etching solution must be discarded, because satisfying etching results cannot be achieved any longer. Further, the precipitate spreads over the entire metallizing process of the plastic surfaces and impairs the metal deposition, so that faulty or other-wise insufficient metal layers are finally deposited. A constant high quality of the metallization therefore cannot be achieved.